1. Field of the Invention
This invention relates generally to current variable shunt impedances with non-linear control characteristics and specifically to such an impedance intended to functionally imitate and replace a control voltage responsive field effect transistor commonly employed in the Dolby B noise reduction system.
2. Description of the Prior Art
Added quality in reproduced sound has long been important to the ever-growing ranks of home high fidelity enthusiasts. Twenty years ago, high-fidelity sound equipment for home use was produced by relatively few manufacturers for a limited market. Today, a host of manufacturers vigorously compete with extensive product lines to satisfy diverse segments of a large and complex market. The directions in which the consumer high-fidelity sound equipment industry grew were dictated in large measure by a few significant technical innovations. The most important of these innovations have brought improved sound quality at modest cost to large segments of the public. For example, the 33 331/3 rpm long-play record greatly improved the quality of recorded sound compared to the older 78 rpm records and spawned a new generation of manual and automatic turntables.
Pre-recorded reel-to-reel magnetic tapes and simultaneous AM/FM radio broadcasts provided the first widespread public opportunity for high-fidelity enthusiasts to sample and enjoy the added dimension of stereophonic sound. Stereo was warmly received by consumers and soon after its introduction a host of two-channel amplifiers and radios designed to receive AM and FM broadcasts simultaneously appeared. Pleasant though this early stereophonic sound may have been, the cost of pre-recorded reel-to-reel magnetic tape was high and the fidelity available with AM broadcasting techniques was limited.
The introduction of stereophonic records brought the cost of stereophonic sound within the easy reach of much of the public. Monophonic compatible FM multiplex broadcasting techniques were developed and standarized by the Federal Communications Commission. These techniques allowed two distinct stereophonic signals to be combined, broadcast simultaneously and separated electronically by AM radio equipped with a multiplex adapter. These developments in recording and broadcasting caused a wide variety of new high-fidelity sound equipment to be produced. However, they did not end the quest for quality in sound reproduction. Although stereo records are capable of providing excellent fidelity when new, they tend to develop surface noise with prolonged use and are limited in that they can only be played on stable, stationary equipment. Similarly, FM multiplex radios are also capable of providing excellent fidelity under optimum conditions. However, with weak signals, an unfavorable signal to transmission noise ratio results in the presence of objectionable carrier noise.
About ten years ago, the now ubiquitous tape cassette was introduced. Although not originally intended as a high fidelity medium, its popularity as a medium for portable tape recorders, dictating equipment, telephone answering equipment, selected digital systems applications and the like was predictable. The relatively slow tape transport speed of 12/3 inches per second was originally though too slow to provide adequate frequency response for high fidelity sound applications. Additionally, the high level of tape hiss associated with casettes was also viewed as a significant limitation. Nevertheless, the relative low cost and high convenience of the cassette combined with its ability to be used in portable or mobile equipment, such as car stereo tape players, encouraged record manufacturers to offer prerecorded selections on cassettes.
In the late 1960's work was in progress on the development of novel signal compression and expansion techniques for reducing noise, such as the hiss associated with magnetic tape recording. The idea of signal compression and complementary expansion can be used to reduce noise in a wide variety of signal transmission applications. A specific noise reducing signal processing system was developed for consumer audio products and became known as the Dolby B system. This system is substantially shown and described in U.S. Pat. No. Re. 28,426 reissued May 20, 1975 to R. M. Dolby and entitled "Signal Compressors and Expanders." A more general teaching of noise reduction art can be found in U.S. Pat. No. 3,846,719 issued Nov. 5, 1974 to R. M. Dolby and entitled "Noise Reduction Systems."
When the Dolby B noise reduction system is used in combination with a high quality cassette recorder and magnetic recording tape possessing low noise and high output characteristics, such as for example, chromium dioxide tape, sound quality almost indistinguishable from that obtainable with new 331/3 rpm long-play records is achieved with all the advantages associated with cassettes. Many manufacturers now produce such cassette tape decks equipped with Dolby B circuitry and these machines sell well in the marketplace.
Once a signal has been compressed (encoded) and recorded according to Dolby B standards, a complementary expander (decoder) must be employed to recover an undistorted signal. However, to the uncritical ear, the only noticeable defect in the compressed signal is an undue emphasis of high frequencies. This emphasis can be approximately corrected by a simple, passive treble cut circuit which acts upon the entire signal. This feature allows Dolby B encoded audio signals to be compatible with audio equipment not provided with Dolby B decoding circuitry.
The Dolby B noise reduction system has performed so well in reducing tape hiss that the Federal Communications Commission has recently authorized its use to reduce transmission noise in FM broadcast signals. As mentioned above, a signal encoded in accordance with the Dolby B standards can be listened to without decoding. Therefore, Dolby B encoded FM broadcast signals are compatible with existing FM and FM stereo radios.
One major obstacle confronting equipment manufacturers who wish to incorporate the Dolby B noise reduction system in new tape recorders, FM radios and the like is the nature of the now-standardized complementary encode/decode characteristics. These standards were previously established using noise reduction systems fabricated from discrete circuit components. Consumer audio equipment manufacturers currently have two choices in implementing Dolby B noise reduction in their products. Discrete component circuitry can be employed which provides perfect matching with existing performance standards. However, this choice is expensive, space consuming and requires initial system adjustments. Alternatively, manufacturers can elect to use the Signetics Integrated Circuit (IC) part No. NE545. When compared to discrete components, the Signetics IC offers the advantages of lower cost, increased reliability, lower space requirements and absence of initial adjustments. However, the encode and decode characteristics of the Signetics part do not conform closely under both static and dynamic conditions with established Dolby B standards. This failure to match the established standards introduces distortions when a properly encoded signal is decoded. These distortions will occur, for example, when pre-recorded cassettes are played or when properly encoded FM broadcast signals are decoded.
As will be described in more detail hereinbelow, the basic Dolby B circuit uses a variable frequency filter controlled by a variable resistance element in a high pass filter configuration. In the original commercial embodiment of this system, a discrete junction Field Effect Transistor (FET) was used as the variable resistance element. Control of the FET was achieved by means of a DC control voltage derived from a rectified audio signal. In attempting to match the standardized Dolby B characteristics, the Signetics IC substitutes a current variable shunt impedance for the junction FET. Such an impedance is taught in U.S. Pat. No. 3,761,741 issued Sept. 25, 1973 to Werner H. Hoeft and entitled "Electrically Variable Impedance Utilizing the Base Emitter Junctions of Transistors."
As a practical matter, many manufacturers of intermediate quality tape recorders and FM radios have found the distortions introduced through the use of the Signetics IC far outweighed by its associated advantages and have incorporated this part in their products. Nonetheless, manufacturers of high quality tape recorders and FM radios must still incorporate the Dolby B noise reduction circuitry in costly discrete component form.